As having good stiffness, transparency and moisture resistance, films such as biaxially-stretched films are widely used as wrapping films. Such films, however have a low heat-sealability. Therefore, for such wrapping films, preferred are multi-layered films having a sealant layer of a resin with good heat-sealability at low-temperature on one or both surfaces, to films of propylene homopolymers with poor heat-sealability at low-temperature.
For T die-cast films of crystalline propylene polymers which are widely used as wrapping films for bread, also preferred are laminated or co-extruded multi-layered films having a sealant layer of a resin with good heat-sealability at low-temperature on one or both surfaces.
In addition to their good low-temperature heat-sealability and high strength after heat sealing, films are further required to have good slipping and anti-blocking properties in the step of re-winding the films being produced, and also to have good outward appearance and good transparency. Moreover, desired are resins capable of being formed into high-quality films even in recent high-producible, large-scale sheeting machines and high-speed sheeting machines.
Moreover, since the resins for such heat-sealing layers are expensive, the recent tendency is toward thinner laminate films having higher stiffness while exhibiting much higher heat-sealing strength. In order to meet those requirements, not only the resin characteristics of heat-sealing layers but also those of substrate layers have been improved. For example, one attempt is to copolymerize propylene with ethylene, 1-butene or the like comonomer to give resin substrate layers, which, however, is still problematic in that the copolymerization greatly lowers the crystallinity of the resins formed and therefore the resins could not have good stiffness.
On the other hand, in general, wrapping films are formed into bags through heat sealing, then the bags are charged with contents and closed also through heat sealing, and thereafter final consumers open the sealed bags to take out the contents. Thus, the process from the sheeting of those wrapping films to the end of their role takes a long period of time. Therefore, wrapping films are required to maintain their good quality, especially impact resistance, for the necessary period of time and under ordinary temperature conditions. Unfortunately, however, no example is known at least at present that is capable of satisfactorily removing the unfavorable phenomena of wrapping films, such as the reduction in their impact resistance and the reduction in their transparency.